Recently, with the rapid increase in multimedia services over LTE mobile communication networks, optical communication networks are increasingly used as backbone networks for the mobile communication networks. Also, Fiber-To-The Home (FTTH) rapidly increases with the increase in multimedia services at home, such as a Video-On-Demand (VOD) service. Meanwhile, as multimedia services based on such optical fiber cables increase, service providers need to continuously check the states of a great number of optical fiber cables dispersed over wide areas, and in the event of a failure, they must restore optical fiber cables by immediately finding the location where a fault occurs.
As a typical optical measuring instrument for checking optical fiber cables and for finding the location where a fault occurs, there is an Optical Time Domain Reflectometer (Hereinafter, called OTDR) using optical pulses. In an OTDR, a laser 100 inputs an optical pulse, having a large amplitude and a short pulse width, to an optical fiber cable and receives an optical pulse reflected from the cleaved fiber ends or an optical fiber connector, as illustrated in FIG. 1, whereby the result shown in FIG. 2 may be obtained. Then, the state of the optical fiber cable may be estimated by analyzing the received optical pulse. Because the principle of how an OTDR operates corresponds to known technology, a detailed description thereabout will be omitted.
(Reference: Korean Patent Application Publication No. 2004-23305 and Korean Patent No. 199128648)
A conventional OTDR using optical pulses is useful in the management of the quality of optical fiber cables but has the following disadvantages.
First, it is difficult to increase the dynamic range thereof. Here, the dynamic range means the distance measurable using the OTDR, and it is necessary to increase the amplitude of an optical pulse in order to increase the dynamic range. However, if the amplitude of the optical pulse is increased so as to be equal to or greater than the threshold thereof, a strong nonlinear effect arises from the interaction between an optical fiber cable and an optical pulse, and the shape of the optical pulse is distorted, thus causing measurement error. In order to avoid such error, currently, the length (width) of an optical pulse is increased, rather than increasing the amplitude of the optical pulse. In this case, the dynamic range may be increased. However, with the increase in the length of an optical pulse, the resolution of the OTDR decreases, as shown in FIG. 3. The shorter the length of an optical pulse, the higher the resolution. Resolution is represented using parameters such as an event dead zone, an attenuation dead zone, and the like. Because these features are associated with each other, if one feature is enhanced, the other feature may be degraded.
Also, as another method for increasing the dynamic range, an optical amplifier (for example, an Erbium-doped fiber amplifier (EDFA)) may be used, but because the conventional OTDR uses a high-power optical pulse, the optical intensity of which quickly changes over time, it is inappropriate to use the EDFA to amplify the optical pulse. As described above, the conventional art has limitations in improving the dynamic range and resolution, and thus technology for solving these problems is required.